There has been proposed an optical disc such as a high-definition videodisc (HDVD) in which a high-definition picture is recorded. Since the HDVD uses a large spectrum space which is four times as much as the conventional NTSC format videodiscs, the video signal is divided into a plurality of components which are retarded in different tracks of the disc. The high-definition video signal is obtained by simultaneously reading each track and synthesizing the signals reproduced from the tracks.
Referring to FIG. 5, the videodisc has a plurality of outwardly spiraling tracks on the recording surface thereof. In the example, the tracks comprises three adjacent tracks 11, 12 and 13. Namely, the track 11 extends to the track designated by references 11a and 11b, after one turn and two turns. Similarly, the tracks 12 and 13 extend to tracks 12a to 13b. The three tracks are simultaneously radiated with beams 4, 5 and 6 So as to be read at the same time. For the. convenience of explanation, it is assumed that the three tracks, are arranged in a group.
The beams 4, 5 and 6 reflected on the tracks are received by three photodetectors, the outputs of which are added to obtain a reproduced RF signal. In the conventional disc player, the distances between the beams 4 and 5, and 5 and 6 in the radial direction of the disc are equal to the distance between the tracks in the radial direction of the disc.
The tracks on the disc can be deviated due to a warp of the disc. In order to properly centralize the laser beams on the tracks, there is provided a track-following servo system where a tracking error signal, which represents the deflections of the beams from the tracks, is obtained from the output signals of one of the photodetectors.
One of the methods for obtaining the tracking error signal is a push-pull method. The push-pull method uses beams split by diffraction.
Referring to FIG. 6, a photodetector 1 for receiving a center beam spot has two detector elements 3a and 3b divided by a center line 2. The beam 5 is diffracted to form a center zeroth-order diffraction beam and, first-order diffraction beams deflected in positive and negative directions from the center zeroth-order diffraction beam.
The center beam and the deflected side beams form an overlapped beam spot on the detector elements 3a and 3b. When the laser beam 5 is properly centralized on the track, the areas of the beam spot on each of the detector elements 3a and 3b are equally distributed. On the other hand, if the tracking is off-center, the reflected beams are asymmetrically diffracted. As a result, the outputs of the detector elements 3a and 3b of the photodetector 1 are different from each other. A track-following servo system is operated to adjust the optical system so that the outputs of the detector elements 3a and 3b become equal to each other. Since the positions of the beams 4 and 6 are set with respect to the center beam 5, when the beam 5 accurately follows the track, so do the beams 4 and 6.
In order to obtain an accurate tracking error signal, the center line 2 defining the detector elements 3a and 3b of the photodetector 1 must be precisely adjusted in the order of microns, at the center of the beam spot. In addition, there is a problem of a DC offset inherent in the push-pull method.
In the push-pull method, an objective of the optical system is moved so as to coincide the center of the laser beam 5 with the track. The spot of the reflected beam formed on the detector elements 3a and 3b is inevitably deflected in the direction perpendicular to the center line 2. Thus, although the size of the overlapped areas on the detector elements 3a and 3b are equal to each other, there is a difference between the outputs thereof due to the difference in the areas. Hence, a DC offset is caused in the tracking error signal. The track-following servo system is operated in accordance with the erroneously offset tracking error signal so that the beams are further deflected from the tracks.
Furthermore the optical disc may be warped so that the optical axes of the laser beams are not perpendicular to the recording surface of the videodisc. The spot on the detector elements 3a and 3b is distorted instead of forming a circle, so that the distribution of energy on the detector elements 3a and 3b differs. Hence, a DC offset is also present in the tracking error signal.
Moreover, when playing an HDVD, the three beams may not be following the right group of tracks, that is, the three tracks being simultaneously read may not belong to the same group. In order to confirm that the correct tracks are read, it is necessary to demodulate the reproduced RF signal, and to determined whether the content of the demodulated RF signal represents a correct signal. The process requires time, which means a delay of starting the reproduction of the disc.